Along with the development of communication technologies, the applications of the WDM have gradually turned from long distance communications to Metropolitans. Due to transparent features of the optics, the WDM can provide bearer channels for many services, e.g. Asynchronous Transfer Mode (ATM), Internet Protocol (IP), Synchronous Digital Hierarchy (SDH) and Synchronous Optical Networking (SONET), etc., therefore, it is usually found that, many non-SDH, non-SONET services are transmitted directly on a certain wavelength of the WDM. However, unexpected malfunctions on optical cables will emerge due to diversified city constructions, e.g. road constructions, pipeline constructions, and etc. Self-Healing Network schemes are adopted for the SDH and the SONET services on the SDH and SONET devices, which can effectively avoid the negative influence resulting from the optical cable failures on the services. However, the non-SDH, non-SONET services, the deficiency of protection measures usually result in service lost for a long time after the optical cable breaks off, which directly brings huge economical losses and pressures to the operators. Therefore, in order to improve the survivability and the reliability of the network, protection and recovery schemes are usually adopted in the WDM system.
In the Metropolitan WDM applications, in order to improve the overall survivability of the network, several Optical Add-Drop Multiplexing (OADM) sites are usually combined into a ring network. The survivability of the WDM ring network is improved through protection methods which are similar to those in the SDH ring network. Common protection schemes in the WDM ring network include Optical Unidireactional Path Switched Ring (OUPSR), Optical Bi-directional path-switched ring (OBPSR), Optical Unidireactional Line-Switched Ring (OULSR), Optical Bi-directional Line Switched Ring (OBLSR) and Optical Sub Network Connection Protection (OSNCP). All these methods have their own advantages and disadvantages. For instance, although both of the OUPSR and the OBPSR can improve the network survivability, their wavelength utilization efficiencies are relatively lower. The familiar OBLSR can properly improve the wavelength utilization efficiency, however, due to its circular protection characteristic, signals have to be transmitted over a long path, and therefore, it is necessary for the system to reserve sufficient margins in Optical Signal to Noise Ratio (OSNR) budgets, which directly limits the transmission specs of the system. As to the OULSR, it has a low wavelength utilization efficiency, and simultaneously, disadvantages of the OBLSR still exist in the OULSR, so it is not very suitable for the applications of the WDM ring network; the OSNCP is usually called as dual transition selective reception protection or 1+1 Optical Transformation Unit (OTU) protection in the WDM ring network, the wavelength utilization efficiency is also relatively lower for the reason that working wavelengths and backup wavelengths take different paths.
At present, based on the OBPSR, there appears a new protection scheme—an Optical Channel Shared Protection Ring (Och-SPRing), which can be described as follows: a same bi-directional service connection is born by a same pair of optical signals with wavelengths of λ1 and λ2 on different segments of a topological ring. The pair of the optical signals λ1 and λ2 are transmitted in two different optical fibers. Their wavelengths are taken as working wavelengths. In addition, another corresponding pair of wavelengths λ1 and λ2 in the two optical fibers is taken as protection wavelengths for the working wavelengths λ1 and λ2. Under normal conditions, the working wavelength is used to bear services, therefore, the working wavelength is also called a working channel, while the protection wavelength functions as a backup, and it is called as a backup channel. The scheme allows to bear various bi-directional connections using the same wavelength pair on different segments of the ring, and the same wavelength pair, which implements the different connections, can use the same pair of backup wavelength channels as the backup channels, and thereby improve the wavelength utilization efficiency. Moreover, all the switchings of the Och-SPRing occur directly between transmit ends and receive ends without ring back, therefore, the system can make its OSNR budget simply based on long paths, which avoids too much margins reserved for the system, and improves the transmission specs.
Most of the existing Och-SPRings are implemented by cross connection units, which means that, demultiplexing the optical signal into several optical wavelength signals and then connecting to the Optical Space Division Cross Connection unit together with local signals, while at the other side, connecting with local downstream signals and a subsequent Optical Signal Multiplexing unit, and completing the re-combination of the optical channels through the cross connection in the middle, and thereby achieving the objective of sharing optical path protection. However, the method needs to demultiplex the optical signals in a lump, and employs the cross connection unit in the course; therefore, it increases the optical power budget of the signals and deteriorates the OSNR. On the other hand, it results in exorbitant costs for initial constructions of the network. However, at early stage of its operation, operators usually require higher efficiency and reliability of the network instead of network capacities. Therefore, the large and all-inclusive protection scheme can hardly meet the practical requirements of the operators.